1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a multi-domain structure implemented with a beam steering film.
2. Discussion of the Related Art
Research has been conducted on various planar panel display devices such as a liquid crystal display (LCD) device, a plasma display panel (PDP), an electroluminescent display device (ELD) and a vacuum fluorescent display device (VFD). Of these devices, the LCD device is used mostly in mobile image display devices such as a notebook computer, because the LCD device has excellent image quality characteristics, is light weight and compact, and has low power consumption characteristics. In addition, the LCD device is also used as a TV monitor or Computer monitor.
In addition, the LCD device displays an image by adjusting the birefringence of a liquid crystal layer interposed between two polarization plates, and by changing the transmittance according to the birefringence. Further, the LCD device is designed to provide an optimal condition with respect to light transmitted in a normal direction of a screen. Thus, the LCD device does not provide an optimal viewing condition when looking at the LCD device from a skewed viewing angle.
In addition, commercialized LCD modes including a twisted nematic (TN) mode, an in-plane switching (IPS) mode, a multi-domain vertical alignment (MVA) mode, an optically compensated bend (OCB) mode, a fringe-field switching (FFS) mode and an electrically controlled birefringence (ECB) mode use rod-like liquid crystals. However, the viewing condition at an angle is even worse for rod-like crystals because of an asymmetry of the birefringence. The poor viewing condition includes a deterioration in contrast and luminance, a color shift, a gray inversion phenomenon, etc.
Turning now to FIGS. 1A and 1B, which are overviews illustrating a concept of the above-mentioned TN mode. As shown in FIG. 1A, the liquid crystals in the TN mode are maintained horizontal to an alignment layer when the power is turned off. Then, as shown in FIG. 1B, when the power is turned on, the liquid crystals around the center of the liquid crystal layer are aligned vertical to the alignment layer in response to an electric field. In addition, a TFT LCD device includes a normally white mode, which is a display mode using an Off-state as white and an On-state as black.
Also, even though the LCD is advantageous because it has a high transmittance and is easy to produce, the TN mode is disadvantageous because of the gray inversion phenomenon that occurs at upper and lower viewing angles. In more detail, the gray inversion phenomenon is a phenomenon where an image looks brighter at a darker gray scale than at a brighter gray scale. In the TN mode, the gray inversion phenomenon makes an image look bright at an upper viewing angle and look dark at a lower viewing angle.
Further, the gray inversion phenomenon is worse at a lower viewing angle, which deteriorates a screen quality of the TN mode LCD and also limits the utility of the TN mode LCD. FIG. 2 is a graph illustrating the gray inversion phenomenon according to upper and lower viewing angles of the TN mode LCD device. Also, the most significant cause of the gray inversion phenomenon is a variation in refractivity according to the viewing angles.
In more detail, and as shown in FIGS. 3A and 3B, the TN mode exhibits a small variation (dΔn⊥≈dΔn1≈dΔn2) of the refractivity according to the viewing angles in the Off-state, and a significant variation (dΔn⊥≠dΔn1≠dΔn2) of the refractivity according to the viewing angles in the On-state. The variation occurs because in the On-state an average director of the liquid crystals is slanted in the upper and lower directions, causing the light passing through the liquid crystals to experience an actual birefringence property (effective dΔn) which changes according to the viewing angles. Further, this phenomenon is more significant in the upper and lower directions.
In more detail, and as shown in FIG. 4, dΔn of a dark gray scale becomes greater than dΔn of a bright gray scale at a lower viewing angle below an apex where Δn of the average director of the liquid crystals theoretically becomes zero (0). Further, dΔn of the dark gray scale becomes greater than dΔn of the bright gray scale at an upper viewing angle above an apex where Δn of the average director theoretically becomes the maximum value. This phenomenon is exhibited as a gray inversion on a screen of a liquid crystal panel. In addition, the reference numerals 30 and 31 indicate upper and lower substrates, respectively, and the reference numeral 40 indicates a liquid crystal panel in FIGS. 3A to 4.
One method to overcome the gray inversion phenomenon is to dispose two or more domains of liquid crystals in a single pixel with a primary viewing angle of the liquid crystals of one domain being directed opposite to that of the liquid crystals of the other domain, thereby compensating the domains with each other. This method uses the counterbalancing asymmetry of the viewing angles so the asymmetric viewing angles are provided in opposite directions.
For example, FIGS. 5A, 5B and 6 illustrate multiple domains in the TN mode LCD device. As shown in FIGS. 5A, 5B and 6, a user senses a mixed light passing through a first domain and a second domain. In other words, because the user senses an average value of dΔn of the first domain and dΔn of the second domain, it is possible to compensate the optical asymmetry. Further, the reference numerals 50 and 51 in FIGS. 5A, 5B and 6 correspond to upper and lower substrates.
Thus, in the TN mode, a multi-domain including two domains in the upper and lower directions is mainly effective, because the asymmetry of the viewing angles occurs mainly in the upper and lower directions. Further, the asymmetry of the viewing angels and the gray inversion phenomenon are less in the right and left directions compared with the upper and lower directions because of a self compensation of the TN mode and a wide viewing angle in the right and left directions.
Further, as shown in FIG. 7, when using four domains (first to fourth domains) in upper, lower, right and left directions, one would expect to have an even more uniform improvement of the viewing angles in the four directions than the multi-domain effect in the right and left directions. However, the multi-domain has not been commercialized because a rubbing process must be performed differently for each domain. Furthermore, a wide view film used to widening the viewing angle cannot be applied to the TN mode.
In addition, the VA mode is used as a wide viewing angle mode to thereby solve the problem related to the viewing angle using the multi-domain. However, even in a four domain VA mode such as a multi-domain vertical alignment (MVA), a pattern-domain vertical alignment (PVA), etc, the viewing angle characteristic is not perfect. In particular, FIG. 8 illustrates that for a VA mode LCD, the gray characteristics are changed according to the viewing angles such that a gray scale in the front side is different from a gray scale at a skewed angle.
A pixel division driving method (S-PVA) has been suggested to improve this problem, but the method still does not completely solve the problem. In addition, as shown in FIGS. 9A and 9B, the optical characteristics at a skewed angle are also significantly deteriorated compared to the front side in terms of luminance and contrast of the MVA.
In addition, the IPS mode and the FFS mode exhibit the best viewing angle characteristics among the modes used in commercialized LCD devices, and thus exhibit the least variation in optical characteristics according to the viewing angles. However, these modes still exhibit a deterioration in the luminance at the skewed angle. In particular, and as shown in FIGS. 10 and 11, the IPS mode uses a multi-domain to solve a color shift problem according to the viewing angles. The reference numerals 100 and 110 correspond to common electrodes, and the reference numerals 101 and 111 correspond to pixel electrodes in FIGS. 10 and 11. However, the multi-domain implemented in the IPS mode does not solve the problem of color shift in a black state. Rather, the IPS mode must use an expensive compensation film to solve this problem.
For example, FIG. 12 shows a contrast of the IPS mode free from the compensation film according to the viewing angles. As shown, the optical characteristics are deteriorated at the skewed angle compared with those in the front side.
In summary, the related art liquid crystal display device has the following problems. The TN, VA and IPS modes have a deteriorated contrast and luminance, and a color shift and/or gray inversion phenomenon at skewed angles. Further, it is difficult to commercialize the method of using a multi-domain because of the complex production and costs related to the multi-domain method.